Low-protein diet therapy for various kidney diseases (for example, chronic kidney diseases, acute glomerulonephritis, and acute kidney injury) has been practiced for a long time. Its mechanism of action has not still been clarified, but is thought to (1) reduce the total amount of the nitrogen compounds derived from protein, and decrease the glomerular loading, (2) suppress the production of uremic toxins derived from protein, which cause renal injury, (3) suppress the accumulation of phosphorous or potassium derived from protein, (4) suppress the production of acids derived from protein, and the like, by inhibiting the intake of the protein derived from diet. The effect of the low-protein diet therapy on inhibiting the progression of the kidney diseases has been proved in the clinical tests that have hitherto been conducted ((a) “The New England Journal of Medicine”, 1989, Vol. 321, No. 26, pp. 1773-1777; and (b) “American Journal of Kidney Diseases”, 2003, Vol. 41, No. 3, pp. S31-S34), and the intake amount of the protein for a patient with a kidney disease is also established in society guidelines (Japan Society of Nephrology, “Evidence-Based Clinical Practice Guideline for CKD 2013”, 2013, pp. 25-30). On the other hand, the low-protein diet therapy has problems of a low extent of long-term strict practice due to necessity for technical knowledge, high cost, and low dietary compliance resulting from taste.
It is known that a compound which inhibits trypsin as one of serine proteases is useful for diseases involving this enzyme, such as pancreatitis and gastroesophageal reflux disease. Indeed, camostat mesylate (which will be hereinafter described Camostat) of the following Formula (A) which is a trypsin inhibitor (Patent Document 1) has been actually used for chronic pancreatitis and gastroesophageal reflux disease in clinical practice. Further, it has also been reported that Camostat has effects of inhibiting the urinary albumin excretion in animal models with diabetes mellitus (“Nephron”, 1996, Vol. 74, No. 4, pp. 709-712), and reducing the amount of the urinary protein excretion in a variety of kidney diseases patients (“Clinical Nephrology”, 1989, Vol. 32, p. 119-123).

In addition, trypsin is involved in the proliferation of influenza viruses. For example, it is necessary that hemagglutinin (HA) on the virus surface should be cleaved into two subunits of HA1 and HA2 by the trypsin in the airway or mucosal intestinal epithelium in order to obtain the infectivity of the virus. It has been reported that by inhibition of the trypsin, the cleavage of this HA is suppressed and the virus loses infectivity, whereby the proliferation is suppressed. Therefore, a compound inhibits the trypsin can also be used as an anti-influenza drug ((a) “Antiviral Research”, 2011, Vol. 92, No. 1, p. 27-36; (b) “Protease Groups of Individuals which Determine Susceptibility to Infection of Influenza Virus and Pathogenesis of Influenza-Associated Encephalopathy”, “The Japanese Journal of Pharmacology”, 2003, Vol. 122, p. 45-53).
As a compound exhibiting a trypsin inhibitory activity, other than Camostat, Compound (B) (Patent Document 2), Compound (C) (Patent Document 3), Compound (D) (Patent Document 4), Compound (E) (Patent Document 5), Compound (F) (Patent Document 6), Compound (G) (Patent Document 7), and Compound (H) (Patent Document 8) of the following formulae have been reported. However, there is no disclosure of the compound of the formula (I) or a salt thereof of the present application as described later in these documents.

(In the formula (C), A is —N(R1, R2) or the like, and R1 and R2 are each H, lower alkyl having 1 to 8 carbon atoms, aralkyl which may have a substituent, or the like. In the formula (D), X is an oxygen atom or a sulfur atom, and R is —C(O)N(R1)—(CH2)m-(1-azabicyclo[3.3.0]octan-5-yl) or the like. In the formula (E), R1 is a hydrogen atom or a halogen atom, and R2 is —OCOR3 or the like. In the formula (F), A is (CH2)n or a styrene group, R2 is —NH(CH2)mCOOR4, —NHCH—(—R5)—COOR4, —NH—C6H4—(CH2)p—COOR4, or the like, m is 2 or 3, p is an integer of 0 or 1, R4 is a hydrogen atom, lower alkyl, or a substituted or unsubstituted benzyl group, R5 is a substituted or unsubstituted benzyl group, a methoxycarbonylmethyl group, and the substituent of the substituted benzyl group means a halogen atom, a nitro group, a lower alkyl group, a hydroxy group, an alkoxy group having 2 to 6 carbon atoms, or the like. In the formula (G), X represents lower alkylene or the like, Y represents a carbonyl group or the like, A represents —NR6R7, R6 and R7 may be the same as or different from each other and each represents a hydrogen atom, a lower alkyl group which may have a substituent, or the like, or R6 and R7 may be bonded to each other to form a cyclic amino group which may have a substituent. In the formula (H), R2 is a substituted phenyl group, or the like, and R3 is any of various substituents. For the other symbols, refer to the respective patent publications.)
Furthermore, as a guanidino compound having an effect of inhibiting the production and release of inflammatory cytokines, Compound (J) (Patent Document 9) has been reported. However, there is no disclosure or suggestion of a specific compound as the compound of the formula (I) or a salt thereof of the present application as described later in these documents.

(wherein R is a guanidino group, an amidino group, or the like; A1, A2 and A3 are each a bond or the like; L is an arylene group or the like; X is —COO— or the like; M is an arylene group or the like, or a divalent heterocyclic group, which has at least one hetero atom selected from a nitrogen atom, a sulfur atom, or an oxygen atom, and may form a fused ring, or the like; R5 is a hydrogen atom or the like; R6 is —CR12R13—(CH2)m—R11 or the like; R12 and R13 are a hydrogen atom or the like; R11 is —COOR16 or the like; and R16 is a hydrogen atom or the like. For the other symbols, refer to the corresponding patent publications.)
Furthermore, Compound (K) has been reported as a guanidino compound which is useful as a pollen protease inhibitor (Patent Document 10). However, there is no disclosure or suggestion of a specific compound as the compound of Formula (I) or a salt thereof of the present application as described later in this document.

(wherein R2c represents a ZcCONR5cR6c group or the like, Zc represents a bond or the like, R5c and R6c each independently represent a hydrogen atom, an alkyl group having 1 to 4 carbon atoms, an alkyl group having 1 to 4 carbon atoms, substituted with a COOR4c group, a phenyl group, a benzyl group, a pyridyl group, or the like, and R4c represents an alkyl group having 1 to 4 carbon atoms, a phenyl group, or the like).
Furthermore, a guanidinobenzoic acid derivative (L) which is useful as a house dust mite protease inhibitor has been reported (Patent Document 11). However, there is no disclosure or suggestion of a specific compound as the compound of Formula (I) or a salt thereof of the present application as described later in this document.

(wherein R2 represents a phenyl group, a naphthyl group, a substituted phenyl group, or a substituted naphthyl group, and R3 represents one of various substituents).